JP-A-2009-97469 discloses an exhaust purifying system including a selective catalytic reduction NOx catalyst (SCR). This system includes a three-way catalyst (TWC), a storage-reduction type NOx catalyst (NSR), and a selective reduction type NOx catalyst (SCR) disposed in sequence in an exhaust passage toward a downstream side. The NSR has a function of storing NOx contained in an exhaust gas when an air-fuel ratio of an incoming exhaust gas is lean, and reducing the NOx stored when the air-fuel ratio of the incoming exhaust gas is stoichiometric or rich and under the presence of a reducing agent. The SCR has a function of, with the supply of NH3 as the reducing agent, selectively reducing NOx that has leaked downstream the NSR.
The NOx reduction reaction by the SCR with NH3 as the reducing agent is an exothermic reaction and tends to progress more under low temperature environment. The related-art system described above therefore uses temperature control means for keeping a temperature of the SCR at a level equal to, or less than, a predetermined reference temperature. This maintains a condition in which the NOx reduction reaction progresses ideally in the SCR, so that a NOx purification rate by the SCR can be increased.    Patent Document 1: JP-A-2009-97469    Patent Document 2: JP-A-2007-9810    Patent Document 3: JP-A-2009-90273    Patent Document 4: JP A 2008 2797334 JP-A-2008-279334